Modification of temperature dependence of pitch viscosity for carbon article manufacture

ABSTRACT

Methods are provided for reducing or minimizing the temperature dependence of a pitch feed or fraction for use in carbon fiber production, such as a mesophase pitch feed or fraction or an isotropic pitch feed or fraction. A pitch sample can be characterized to determine a characteristic temperature and a characteristic viscosity for the sample. One or more solvent extraction processes can also be performed on the pitch and/or the extract and raffinate fractions formed by the solvent extraction(s). The resulting raffinate and extract fractions are then used to form a modified pitch fraction with a T 0  value that is lower than the T 0  value of the original pitch. The modified pitch fraction can optionally also have a different η inf  value relative to the original pitch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 62/685,571 filed Jun. 15, 2018, which is herein incorporated by reference in its entirety.

FIELD

Systems and methods are provided for modifying the temperature dependence of pitch feedstocks, such as mesophase pitch feedstock and/or isotropic pitch feedstocks, for use in production of carbon fiber.

BACKGROUND

Isotropic pitch and mesophase pitch are carbon-containing feedstocks that can be formed from residues generated during processing of coal or petroleum feedstocks or by other methods, such as acid catalyzed condensation of small aromatic species. As an example, one of the fractionation products from a catalytic cracking process or slurry hydrocracking process can be a bottoms or “pitch” fraction. Such a pitch fraction can correspond to isotropic pitch. For some grades of carbon fiber, isotropic pitch can be used as an initial feedstock. For other grades, the isotropic pitch can be exposed to thermal cracking conditions to cause formation of an anisotropic phase. The anisotropic pitch phase (or mesophase pitch) can be identified, for example, using polarized light microscopy. This mesophase pitch can then be used for carbon fiber production. Optionally, a solvent can be added to isotropic pitch and/or mesophase pitch to modify the viscosity prior to carbon fiber formation.

The production of carbon fiber from isotropic pitch or mesophase pitch is typically achieved by a melt spinning process. The pitch is heated to sufficiently high temperatures to reduce the viscosity of the pitch, so that the heated pitch can pass through a spinnerette. The resulting carbon fiber is then wound on a spinning spool. However, the viscosity of many types of pitch samples is strongly dependent on temperature. This strong temperature dependence can cause large variations in the fiber diameter and/or high tensile stress within the filament with small temperature gradients during fiber formation. In order to overcome this difficulty, conventional production of carbon fiber from pitch requires the process to be operated within a narrow temperature window, which can be challenging to maintain under commercial production conditions. Additionally, difficulties in maintaining the production process in the desired temperature window can also limit the throughput of the produced fiber. For example, the sensitivity of the fiber formation process to small temperature variations can result in fiber breakage. The fiber breakage can be caused, for example, due to loss of ability for the mesophase pitch to flow through the spinnerette and/or due to structural weak points in the fiber that are created by size variations and/or tensile stress. It would be desirable to identify systems and/or methods that can improve the ability to produce carbon fiber from mesophase pitch.

U.S. Pat. No. 4,208,267 describes methods for forming a mesophase pitch. An isotropic pitch sample is solvent extracted. The extract is then exposed to elevated temperatures in the range of 230° C. to about 400° C. to form a mesophase pitch.

U.S. Pat. No. 5,643,546 describes methods for forming carbon fiber from mesophase pitch. As part of the preparation for the meosphase pitch, the mesophase pitch is solvent extracted using a mixture of solvents with different solubility parameters to remove an undesired extract portion.

U.S. Pat. No. 6,717,021 describes the addition of an aromatic solvent as a solvating component to produce a solvated mesophase pitch. While this route decreases the viscosity of the material, it requires sacrificial solvent, dilutes the amount of mesophase pitch produced and introduces volatile components which may break the fiber during spinning.

SUMMARY

In an aspect, a method for forming a modified pitch is provided. The method includes performing solvent extraction on a pitch comprising a T₀ value to form an extract comprising a first fraction of the pitch and a raffinate comprising a second fraction of the pitch. At least a portion of the first fraction can be blended with at least a portion of the second fraction to form a modified pitch having a modified T₀ value less than the T₀ value of the pitch. For example, the modified pitch can have a modified T₀ value that is at least 5.0° C. less than the T₀ value of the pitch.

In some aspects, the at least a portion of the first fraction can be formed from the extract without performing an additional solvent extraction step. Additionally or alternately, in some aspects the at least a portion of the second fraction is formed from the raffinate without performing an additional solvent extraction step.

In some aspects, the at least a portion of the second fraction can correspond to a portion that is formed by performing a second solvent extraction and/or the at least a portion of the first fraction can correspond to a portion that is formed by performing a third solvent extraction.

In another aspect, a method for forming a modified pitch is provided. The method includes performing solvent extraction on a pitch comprising a T₀ value to form an extract comprising a first fraction of the pitch and a raffinate comprising a second fraction of the pitch. A second solvent extraction is then performed on a) at least a portion of the first fraction or b) at least a portion of the second fraction, to form a second extract comprising a third fraction of the pitch and a second raffinate comprising a fourth fraction of the pitch. In some aspects, the second solvent extraction is performed on the at least a portion of the first fraction. In such aspects, a T₀ of the fourth fraction can be less than the T₀ value of the pitch and/or the modified pitch can correspond to at least a portion of the fourth fraction. In some aspects, the second solvent extraction is performed on the at least a portion of the second fraction. In such aspects, a T₀ value of the third fraction can be less than the T₀ value of the pitch and/or the modified pitch can correspond to at least a portion of the third fraction.

In some aspects, the pitch can correspond to a mesophase pitch and the modified pitch can correspond a modified mesophase pitch. In some aspects, the pitch can correspond to an isotropic pitch and the modified pitch can correspond to a modified isotropic pitch.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an example of a process flow for forming a modified pitch fraction from an initial pitch fraction.

FIG. 2 shows an example of fitting an Arrhenius-style functional form to viscosity and temperature data.

FIG. 3 shows a comparison of predicted T₀ and η_(inf) values (determined according to equations 3 and 4) with measured values for various blends of two vacuum resid fractions.

FIG. 4 shows viscosity as a function of temperature for a hypothetical initial mesophase pitch fraction and a modified mesophase pitch fraction.

FIG. 5 shows the derivative with respect to temperature for the viscosity versus temperature data in FIG. 4.

DETAILED DESCRIPTION

In various aspects, systems and methods are provided for reducing or minimizing the temperature dependence of a pitch feed or fraction for use in carbon fiber production, such as a mesophase pitch feed or fraction or an isotropic pitch feed or fraction. Initially, a mesophase pitch sample (or more generally a pitch sample) can be characterized to determine a characteristic temperature and a characteristic viscosity for the sample. Examples of characteristic values for a pitch fraction are the asymptotic values of viscosity at infinite temperature (η_(inf)) and the finite temperature at which the viscosity diverges (T₀). In addition to determining a characteristic temperature and a characteristic viscosity, one or more solvent extraction processes can be performed on the pitch and/or the extract and raffinate fractions formed by the solvent extraction(s). The resulting raffinate and extract fractions are then used to form a modified pitch fraction with a T₀ value that is lower than the T₀ value of the original pitch. The modified pitch fraction can also have a different η_(inf) value relative to the original pitch. By reducing the T₀ value, the temperature dependence of the viscosity for the modified pitch is reduced relative to the initial pitch fraction. Reducing the temperature dependence of the viscosity can reduce or relax the temperature constraints that are needed for producing a desired carbon fiber product from the pitch. Optionally, the blend of extract(s) and raffinate(s) can be selected to both reduce the T₀ value and increase the η_(inf) value. Reducing the T₀ value of a mesophase pitch fraction and/or other pitch fraction can also tend to cause a reduction in the viscosity. Selecting a blend of extract(s) and raffinate(s) that increases η_(inf) can offset some of the decrease in viscosity due to lowering the T₀ value.

In some aspects, the modified pitch fraction (such as a modified mesophase pitch fraction) can have a T₀ value that is less than the T₀ of the initial pitch by at least 5° C., or at least 10° C., or at least 15° C., such as down to 40° C. less or possibly still lower. Additionally or alternately, the modified (mesophase) pitch fraction can have a η_(inf) value that is greater than the η_(inf) of the initial pitch by 0.001 Pa-sec or more, or 0.002 Pa-sec or more, or 0.003 Pa-sec or more, such as up to 0.01 Pa-sec or possibly still greater.

In some aspects, a modified pitch fraction can be formed by performing successive solvent extractions, but without re-blending of the products from the solvent extractions. In such aspects, the successive solvent extractions can be used to form a “heart cut” of the original pitch fraction with a reduced T₀ value. For example, successive solvent extractions can be used to form a modified pitch fraction where the highest solubility parameter components and lowest solubility parameter components in the initial pitch fraction are reduced or minimized. This can be achieved, for example, by performing a solvent extraction on a pitch sample to form an initial extract and an initial raffinate. If the desired portion of the pitch is in the initial extract, a second extraction can be performed using a second solvent, with the second solvent selected so that the desired portion of the pitch is part of the second raffinate. Alternatively, if the desired portion of the pitch is in the initial raffinate, a second extraction can be performed so that the desired portion of the pitch is part of the second extract.

In some optional aspects, a test amount of carbon fiber may be made from a pitch sample prior to separation and/or blending to form a modified pitch. In such aspects, the T₀ and values for the pitch fraction can be determined before or after forming the carbon fiber product. If the temperature dependence of the pitch fraction (such as a mesophase pitch fraction) results in carbon fiber product with undesirable properties, then solvent extraction and blending can be performed to make a modified pitch fraction. For example, for carbon fiber in a green state (i.e., as it leaves the spinnerette and prior to any additional heating or pyrolyzing to make a final carbon fiber product), if carbon fiber made from the pitch has a variation in diameter of 10% or more, or 15% or more, or 20% or more, due to a temperature variation across the fiber during production (i.e., across the face of a spinnerette) of 4.0° C. or less, or 3.5° C. or less, or 3.0° C. or less, then solvent extraction and blending of the resulting extract(s) and raffinate(s) can be performed to make a modified pitch fraction.

Production of carbon fiber represents a potentially high value end use for isotropic pitch and/or mesophase pitch fractions. Unfortunately, the viscosity of many pitch fractions has a strong dependence on temperature in the temperature ranges suitable for carbon fiber formation. This can cause a variety of problems during manufacture of carbon fiber, such as inconsistent product quality and breaking of the fiber during the spinning process. While addition of solvents to the pitch can potentially reduce the temperature dependence of the viscosity, such solvents can also dilute the pitch and/or increase the likelihood of breaking the fiber during the spinning process. In various aspects, the methods described herein allow an improved pitch fraction to be formed without requiring the addition of a solvent that remain in the improved pitch fraction. Instead, the solvents used for the extraction process(es) to form the improved pitch fraction can be recovered and used again to perform subsequent extractions.

In this discussion, some pitch fractions are described using volume ratios of blending components. In some aspects, volume ratios are provided between pairs of blending components in a fraction. Additionally or alternately, when three or more blending components are used to form a mesophase pitch fraction, volume ratios of three or more components may be used, such as a three component ratio in the form of “X:Y:Z”.

In this discussion, the solvating power of a potential solvent for use in solvent extraction can be defined based on the Hildebrand solubility parameter. The Hildebrand solubility parameter is defined as

$\begin{matrix} {\delta = \left( \frac{\Delta \; H_{v}{RT}}{V} \right)^{0.5}} & (1) \end{matrix}$

In Equation (1), ΔH_(v) is the heat of vaporization of the solvent, R is the molar gas constant, T is the temperature in Kelvin, and V is the molar volume. In this discussion, solubility parameter values are based on use of SI units for expressing the quantities in Equation (1). The units for the solubility parameter correspond to (MPa)^(0.5). Some values for the solubility parameter for representative solvents include: isopentane—13.5; n-pentane—14.4; n-hexane—14.9; n-heptane—15.3; cyclohexane—16.8; toluene—18.3; pyridine—21.7; quinoline—22.4; methylene chloride—20.2; and tetrahydrofuran—18.5. Values for the solubility parameter can be found in various on-line or printed references, such as the Material Properties Tables available on-line from “Knovel Solvents—A Properties Database”, or in tables of solubility parameters in references such as the Adhesives Technology Handbook, 3^(rd) Edition (Ebnesajjadz et al., 2014).

Pitch Feedstock and Characterization

In various aspects, isotropic pitch and/or mesophase pitch can be used as a feedstock for formation of carbon fibers. Pitch feedstocks can be formed from a variety of heavy oil and/or heavy hydrocarbonaceous fractions that include a substantial portion of aromatics. Some heavy oil fractions can be suitable without further processing, while other fractions can be at least partially converted to mesophase pitch feeds by heat treatment and/or performing a limited polymerization. Suitable fractions for use as pitch and/or for forming mesophase pitch can include, but are not limited to, heavy oils, coal tar fractions formed during conversion of coal to coke; bottoms fractions from fluid catalytic cracking; steam cracker tar; pitch formed during slurry hydroconversion and/or fixed bed hydroconversion (such as hydroconversion of heavy oils); and/or “rock” fractions generated during solvent deasphalting of a heavy oil. More generally, pitch fractions for formation of carbon fiber can be formed from any of the above sources.

Suitable pitch feesdtocks and/or mesophase pitch feedstocks for formation of carbon fiber can be identified, for example, based on the coke yield of the (mesophase) pitch feedstock if exposed to pyrolysis or coking conditions. For example, suitable feedstocks can provide a coke yield of 40 wt % or more relative to a weight of the feedstock, or 50 wt % or more, or 75 wt % or more, or 85 wt % or more, or 95 wt % or more, when exposed to thermal cracking conditions that include a coking temperature of 500° C. at atmospheric pressure for 15 minutes, according to the method of ASTM D4530.

Another way of defining a feedstock is based on the boiling range of the feed. One option for defining a boiling range is to use an initial boiling point for a feed and/or a final boiling point for a feed. Another option, which in some instances may provide a more representative description of a feed, is to characterize a feed based on the amount of the feed that boils at one or more temperatures. For example, a “T5” boiling point for a feed is defined as the temperature at which 5 wt % of the feed will boil off. Similarly, a “T95” boiling point is a temperature at 95 wt % of the feed will boil. The percentage of a feed that will boil at a given temperature can be determined, for example, by the method specified in ASTM D2887 (or by the method in ASTM D7169, if ASTM D2887 is unsuitable for a particular fraction). A pitch feed or fraction for forming carbon fiber can have a normal atmospheric initial boiling point of about 350° C. or more, or 400° C. or more, and can further have a penetration range from 20 to 500 dmm at 25° C. (ASTM D-5). Alternatively, a feed may be characterized using a T5 boiling point, such as a feed with a T5 boiling point of 350° C. or more, or 400° C. or more, or 440° C. or more. Additionally or alternately, a feed may be characterized using a T50 boiling point, such as a feed with a T50 boiling point of 500° C. or more.

For mesophase pitch fractions, other types of characterization can be based on solubility and optical activity. In some aspects, the optically active portion of a mesophase pitch fraction can correspond to 10 vol % or more of the fraction, or 25 vol % or more, or 50 vol % or more. In some aspects, the amount of quinoline-insoluble content in a mesophase pitch fraction can be 75 wt % or less, or 50 wt % or less, or 30 wt % or less, or 10 wt % or less, such as down to substantially no quinoline-insoluble content. Additionally or alternately, the amount of toluene-insoluble content in a mesophase pitch fraction can be 80 wt % or less, or 60 wt % or less, or 40 wt % or less, or 30 wt % or less, such as down to substantially no toluene-insoluble content. In some aspects, the width of the glass transition temperature for the mesophase pitch can be 40° C. or less, as determined by differential scanning calorimetry according to ASTM D6604 (or according to E1356, if ASTM D6604 is unsuitable for a particular sample).

Other characteristics of suitable pitch feeds or fractions can include, but are not limited to, softening point, metals content, solids content (for pitch feeds formed from, for example, coal tar), heteroatom content (S, N), and viscosity. For example, some suitable feedstocks can have a softening point according to ASTM D3104 of 100° C. to 300° C., or 150° C. to 250° C. Although some feedstocks may not strictly correspond to an asphalt or bitumen fraction, it is believed that ASTM D3104 is suitable for characterizing pitch fractions. Additionally or alternately, the pitch can include 200 wppm or less of metals and/or 200 wppm or less of total nickel, vanadium and iron. Further additionally or alternately, a pitch fraction (such as a feed for forming carbon fiber) can have a viscosity at 280° C. of 20 Pa-sec or less, or 15 Pa-sec or less.

In some aspects, a pitch feed or fraction can include 50 wppm to 1000 wppm elemental nitrogen or more (i.e., weight of nitrogen in various nitrogen-containing compounds within the feed). Additionally or alternately, the feed can include 500 wppm to 60,000 wppm elemental sulfur. Sulfur will usually be present as organically bound sulfur. Examples of such sulfur compounds include the class of heterocyclic sulfur compounds such as thiophenes, tetrahydrothiophenes, benzothiophenes and their higher homologs and analogs. Other organically bound sulfur compounds include aliphatic, naphthenic, and aromatic mercaptans, sulfides, and di- and polysulfides.

Determining Characteristic Temperature and Viscosity Values for Mixtures

In various aspects, the T₀ and η_(inf) values for a potential feed component and/or potential pitch component can be determined based on Equation (2).

$\begin{matrix} {\eta = {\eta_{\inf}e^{\frac{D}{\lbrack{{({T/T_{0}})} - 1}\rbrack}}}} & (2) \end{matrix}$

In Equation (2), η is the viscosity at a temperature T. η_(inf) and T₀ have the definitions provided above. In Equation (2), D is a constant that can be set based on historical data for pitch and/or asphalt fractions. While D can vary in practice, in Equation (2), sufficient accuracy can be achieved by setting D to a constant value that is representative of the η_(inf) values that are typically encountered in pitch blend and/or asphalt blend components. In this discussion, D will be set to a constant value of 7.5. Other potential values for D could be selected while also giving suitable results, such as a constant value for D between about 5.0 to about 15.0, and preferably a constant value for D between about 6.5 to about 10.5.

In Equation (2), based on the use of a constant value for D, such as 7.5, for a given feed and/or pitch component, η_(inf) and T₀ are the two values that need to be fit to experimental data. Thus, at least two measurements of the viscosity η at a temperature T are needed to fit values to η_(inf) and T₀. In practice, additional measured values can be obtained for a given component in order to further improve the fit of Equation (2) to the data.

One option for obtaining viscosity values at different temperature for a component sample is to obtain viscosity values at different temperatures. Preferably, the viscosity values obtained for a component can be obtained using a single measurement device, so that any systematic errors in the measurement will be the same for all measured values on a component. Alternatively, other types of devices for measuring a kinematic viscosity at a temperature can be used to generate a plurality of viscosity values η at a plurality of temperatures T. An example of a suitable measurement method can be ASTM D4402. It is noted that ASTM D4402 is directed to measurements at up to 260° C. If measurements at higher temperatures are desirable, an alternative method suitable for higher temperature measurement can be used instead.

With regard to selecting temperatures for obtaining viscosity values, several factors can be considered. For isotropic pitch samples, the viscosity values for a component can be obtained at temperatures where the component behaves as a Newtonian fluid. For instances where the pitch exhibits non-Newtonian behavior, the viscosity can be obtained by extrapolating to zero shear. For potential feed fractions and/or pitch samples that have a high hardness value, it may be necessary to measure the viscosity at higher temperatures, such as at least about 130° C., or at least about 150° C., or at least about 200° C., or at least about 250° C., or at least about 300° C., or at least about 350° C. Another factor can be to have at least two data points within the set of data for fitting the η_(inf) and T₀ values that are at least about 10° C. apart. If the temperatures for all of the viscosity values in the data set are grouped too closely together, the quality of the parameter fit may be reduced.

After obtaining two or more viscosity and temperature data point pairs for a potential feed and/or pitch component (such as a mesophase pitch component), the η_(inf) and T₀ can be calculated by fitting the data points to the functional form shown in Equation (2). This process can be repeated for each desired pitch component that is to be included in a pitch blend. Additionally or alternately, the η_(inf) and T₀ values for one or more pitch components in a blend can be determined based on fitting Equation (2) to previously obtained or published data for a component. This previously obtained or published data can correspond to viscosity and temperature values as described above, or the prior data can correspond to other data that allows for calculation of a viscosity at a plurality of temperatures.

One variation on the model is that the glass transition temperature (T_(g)) for a group of components to be used in a feed and/or pitch can be used in place of T₀ if desired. The procedure for fitting measured data to Equation (2) and subsequent use of the T_(g) and η_(inf) values is otherwise similar.

Predicting Pitch Properties Based on Blend Components

Based on T₀ and η_(inf) values for a plurality of components, the T₀ and η_(inf) value for a resulting blend of components can be determined. An advantage of using T₀ and η_(inf) to characterize the components of a pitch blend is that the values can be used in a weighted average to determine the T₀ and η_(inf) value for the resulting blend. In particular, the T₀ value of a blend of components can be determined by Equation (3). The η_(inf) value of a blend of components can similarly be determined by Equation (4).

$\begin{matrix} {T_{0} = {\sum\limits_{i}\; {\varphi_{i}T_{0,i}}}} & (3) \\ {{\ln \mspace{11mu} \eta_{\inf}} = {\sum\limits_{i}\; {\varphi_{i}\ln \mspace{11mu} \eta_{\inf,i}}}} & (4) \end{matrix}$

In Equations (3) and (4), Ø_(i) corresponds to the volume percent of component “i” in a blend. As shown in Equations (3) and (4), the T₀ values can be combined as a weighted average of the T₀ values of the components, while the η_(inf) values can be combined as a weighted average of a log of the η_(inf) values of the components.

Modification of Pitch Fractions

In various aspects, a modified pitch fraction (such as a modified mesophase pitch fraction) can be formed by 1) performing one or more solvent extractions on an initial (mesophase) pitch fraction to form one or more raffinates and one or more extracts; and 2) combining portions of the resulting raffinates and extracts to form a modified (mesophase) pitch fraction having lower T₀ values than the initial pitch fraction.

In some aspects, a modified pitch fraction can be formed using a single solvent extraction stage. In such aspects, an initial pitch fraction is solvent extracted using a suitable solvent to form an extract and a raffinate. The extract substantially corresponds to the portion of the initial pitch fraction that is soluble in the solvent, along with the solvent, while the raffinate substantially corresponds to a portion of the initial pitch fraction that is insoluble in the solvent. Of course, due to practical limitations, the extract may include some insoluble components and the raffinate may include some soluble components, but these can be reduced or minimized to a desired level based on the design and the operating conditions of the solvent extractor. Due to such practical limitations, the extract can be referred to as including a majority of the components from the feed to the extraction stage that are soluble in the solvent, while the raffinate can be referred to as including a majority of the components from the feed to the extraction stage that are insoluble in the solvent.

In other aspects, more than one solvent extraction can be performed. For example, after performing a first solvent extraction, the raffinate from the first extraction stage can be exposed to a second solvent under solvent extraction conditions. The second solvent can have a sufficiently higher solubility parameter value than the first solvent, so that the second solvent is effective for solvating a portion of the raffinate. This process can be repeated as many times as desired to form a plurality of extracts and a final raffinate. Additionally or alternately, the extract from the first extraction stage could be selected for additional solvent extraction steps using solvents with a lower solubility parameter. In this type of aspect, it may be desirable to remove the first solvent from the extract prior to exposing the remaining portion of the extract to the second solvent. In this type of aspect, the second solvent can have a lower solvating power than the first solvent. This process can be repeated as many times as desired to form a plurality of raffinates and a final extract. More generally, in still other aspects, a combination of extraction stages can be used to form a plurality of extracts and a plurality of raffinates. In such aspects, various combinations of using solvents with various solubility parameters can be used to create desired extract fractions and/or raffinate fractions.

A wide variety of solvents are suitable for use in a solvent extraction stage. Typically, suitable solvents can have a T5 to T95 boiling range of 50° C. to 300° C. Examples of suitable solvents include, but are not limited to, C₂-C₁₀ paraffins; single ring aromatics such as toluene, xylene, and ethylbenzene; multi-ring aromatics, such as naphthalene and anthracene; aromatics including a heteroatom such as pyridine; other heteroatom compounds such as tetrahydrofuran; heavy naphtha, kerosene, and/or light diesel fractions; and other hydrocarbon or hydrocarbon-like fractions having a suitable boiling range. Combinations of the above solvents may also be used in order to achieve a desired solubility parameter. In some aspects, a paraffin such as hexane or heptane may be included as a co-solvent to modify the solubility parameter of a solvent mixture. References herein to a solubility parameter are references to the Hildebrand solubility parameter expressed in SI units, as defined above.

In aspects where multiple extractions are performed to form a plurality of extracts and/or raffinates, a first solvent (or a solvent from a prior extraction stage) can have a solubility parameter that differs from a second solvent (or a solvent from a later extraction stage) by 2.0 or more, or 4.0 or more. Additionally or alternately, when multiple extraction stages are used to form a plurality of extracts and a raffinate, in some aspects a suitable solvent for a first stage can correspond to pyridine or another solvent (such as a mixture of solvent components) that has a solubility parameter of 21 or more. In such aspects, a suitable second solvent can have a lower solubility parameter, such as toluene or tetrahydrofuran, or possibly a solvent where at least one component of the solvent corresponds to a paraffin. Such second solvents can have a solubility parameter of 19.5 or less.

After forming the extract(s) and the raffinate(s), the one or more extracts (after removal of solvent) and one or more raffinates (optionally after removing any solvent) can be characterized to determine a T₀ and an η_(inf) value for each portion. Based on the T₀ and an η_(inf) values, the extract and raffinate can be recombined in desired weight ratio(s) to produce a modified pitch product with a lower T₀ value. Any convenient weight ratio of the various extracts and raffinates that results in a lower T₀ value can potentially be used. It is noted that the ratio will be different from the weight ratio(s) of the extract and raffinate that are generated based on the solvent extraction(s).

The desired ratio of extract to raffinate can be determined based on Equations (2) to (4) as described above. The desired ratio can be selected, for example, to form a modified pitch fraction having a T₀ value that is lower than the T₀ value of the initial pitch fraction by 3.0° C. or more, or 5.0° C. or more, or 10° C. or more, or 15° C. or more, such as up to 40° C. or possibly still greater. The amount of difference in T₀ in the resulting extracts and raffinates can be determined in part based on the solubility of the one or more solvents used to form the extracts and raffinates.

Although it is not required, the extract phase (after removal of solvent) will typically have a lower T₀ value than the initial pitch fraction while the raffinate (after removing solvent, if necessary) will typically have a higher T₀ value. As a result, when a single extraction is performed to form a single raffinate and single extract, in many instances the weight ratio of extract to raffinate in the modified mesophase pitch will be greater than the weight ratio of the amount of extract formed versus the amount of raffinate formed during the solvent extraction.

In aspects where a plurality of solvent extractions are performed, at least three fractions will be available for recombination to form the modified mesophase pitch. The three fractions can include the final raffinate, the final extract, and one or more intermediate raffinates or extracts. In some aspects, the one or more intermediate extracts can correspond to extract fractions formed during extraction stages prior to the final stage, with the intermediate extracts not be subjected to further solvent extraction. In some aspects, the one or more intermediate raffinates can correspond to raffinate fractions formed during extraction stages prior to the final stage, with the intermediate raffinates not be subjected to further solvent extraction.

When multiple extracts and/or raffinates are available based on performing a plurality of solvent extractions, multiple options are available for blending the multiple extracts and/or raffinates to form a desired modified pitch with a reduced T₀ value. In some aspects, the composition of the modified pitch can be described in relation to the composition of the initial pitch. The initial pitch feed can roughly be described as a combination of the various extracts and raffinates that are generated by performing solvent extraction(s) on the initial feed, with weight ratios corresponding to the resulting weights of the various raffinate and extract fractions. When a modified pitch is formed by blending portions of the raffinate and extract fractions, the weight ratios for the initial feed can be compared with the weight ratios for the modified pitch. It is noted that for the modified pitch, the value in the weight ratio for one or more of the raffinate or extract fractions may be “0”, as it may be desirable to entirely exclude one or more of the extract and/or raffinate fractions from the modified pitch product.

As an example, one option can be to form a modified pitch having weight ratios corresponding to an increased amount of the final raffinate, an increased amount of at least one intermediate extract, and a decreased amount of the final extract, relative to the weight ratio of the final raffinate, intermediate extract, and final extract in the initial feed. More generally, this can correspond to forming a modified pitch having a weight ratio (relative to the initial feed) corresponding to an increased amount of one or more low solubility portions of the initial pitch (such as the final raffinate), an increased amount of one or more high solubility portion of the initial pitch (such as the initial extract or another intermediate extract), and a reduced percentage of one or more intermediate solubility portions of the initial mesophase pitch (the final extract).

As another example, a modified pitch can have weight ratios corresponding to an increased amount of the final extract, an increased amount of at least one intermediate raffinate, and a decreased amount of the final raffinate. More generally, this can correspond to forming a modified pitch with an increased weight percentage of a high solubility portion of the initial mesophase pitch (such as the final extract), an increased percentage of a low solubility portion of the initial mesophase pitch (such as the initial raffinate or another intermediate raffinate), and a reduced percentage of an intermediate solubility portion of the initial mesophase pitch (the final raffinate).

As will be recognized by those of skill in the art, the above two options can be generally understood as providing a general strategy for forming a modified mesophase pitch having a “dual-peak” composition, where a high solubility and low solubility portion of the initial mesophase pitch are re-combined while incorporating a reduced amount, such as none, of an intermediate solubility portion of the initial mesophase pitch. This type of “dual-peak” composition cannot be formed simply by performing a series of extractions. Additionally, the proper combination of the high solubility and low solubility portions, while reducing or minimizing the amount of the intermediate solubility portion, is enabled by an understanding of how to combine fractions while producing a modified mesophase pitch with a reduced T₀ value.

FIG. 1 shows an example of a process flow for forming a modified mesophase pitch fraction based on performing two solvent extractions using two solvents with different solubility parameters. In the example process flow shown in FIG. 1, a mesophase pitch feed 105 is passed into a solvent extractor 120, along with a first solvent 111. This results in formation of an extract stream 127 that includes a majority of first solvent 111 and a majority of the portions of feed 105 that are soluble in first solvent 111. A raffinate stream 126 including a majority of insoluble portions of feed 105 is also formed. Optionally, a portion of extract stream 127 can undergo a separation (not shown) to separate the first solvent from a first supplemental pitch product 129.

The raffinate stream 126 is then exposed to a second solvent 112 in a second solvent extractor 130. The second solvent 112 can have a higher solvency power than first solvent 111. In some alternative aspects, if a second solvent 112 has a lower solvency power than the first solvent 111, a portion of the extract stream 127 could be used as the input to a second solvent extractor. In various aspects, any convenient number of solvent extractors can be used, each with different solvents, to form a desired plurality of raffinates and/or extracts.

In the example shown in FIG. 1, introducing raffinate stream 126 and second solvent 112 into solvent extractor 130 results in formation of second extract stream 137 and second raffinate stream 136. Second extract stream 137 can include a majority of the components in raffinate stream 126 that are soluble in the second solvent 112, while second raffinate stream 136 can include a majority of second solvent 112 and a majority of components from raffinate stream 126 that are insoluble in second solvent 112.

The outputs from the two solvent extraction stages 120 and 130 correspond to extract 127, extract 137, and raffinate 136. In this type of configuration, a modified mesophase pitch fraction can be formed by selectively combining extract 127, extract 137, and/or raffinate 136 in a desired ratio. In the example shown in FIG. 1, the modified mesophase pitch fraction corresponds to a combination of (at least) a portion 143 of extract 127 and (at least) a portion 146 of raffinate 136. The unused portion of raffinate 136 can correspond to a second supplemental pitch product 139. The portion 143 of extract 127 and the portion 146 of raffinate 136 are blended together in a desired ratio in blending stage 140. Optionally but preferably, at least a portion of the solvent present in portion 143 of extract 127 can be separated from the modified mesophase pitch fraction 145 as a first separated solvent portion 141. The first separated solvent portion 141 can be used, for example, as a recycle stream to form part of first solvent 111. A separation 150 can also be performed on extract 137 to form a second separated solvent portion 152 and a third supplemental pitch product 159. It is noted that an additional portion of second solvent 112 may be included in second raffinate stream 136. This additional portion of second solvent 112 can be recovered as an additional solvent portion 142 by separation as part of blending stage 140. This additional solvent portion 142 can, for example, be combined with second separated solvent portion 152 for recycle and use as part of second solvent 112.

After forming a modified pitch, the modified pitch can be used to form carbon fibers, such as by using a conventional melt spinning process. Melt spinning for formation of carbon fiber is a known technique. For example, the book “Carbon-Carbon Materials and Composites” includes a chapter by D. D. Edie and R. J. Diefendorf titled “Carbon Fiber Manufacturing.” Another example is the article “Melt Spinning Pitch-Based Carbon Fibers”, Carbon, 27(5), p 647, (1989).

Example 1—Determining Characteristic Temperature and Characteristic Viscosity

Characteristic temperature values corresponding to T₀ values and characteristic viscosities corresponding to η_(inf) values were determined for three representative vacuum resid fractions. The dots in FIG. 2 correspond to measured values for viscosity at various temperatures. It is noted that the vertical axis in FIG. 2 corresponds to the log of the viscosity. The lines in FIG. 2 correspond to fits of the measured values to Equation (2), the Arrhenius-style equation for the viscosity-temperature relationship as described above. As explained in association with Equation (2), D was set to 7.5 for purposes of fitting the curve to the data. As shown in FIG. 2, the form of Equation (2) provides a relatively good fit for the shape of the data for each of the vacuum resid fractions. Table 1 shows the resulting T₀ and η_(inf) values determined based on the curve fitting.

TABLE 1 Characteristic Temperature and Viscosity Parameters Sample T₀ (K) η_(inf) (Pa*s) VR1 249.7 8.4E−5 VR2 217.6 4.2E−5 VR3 211.4 2.0E−5

Thus, Equation (2) in combination with measured viscosity values provides a suitable method for determining a characteristic viscosity and a characteristic temperature for a hydrocarbon or hydrocarbon-like fraction.

Example 2—Prediction of T₀ and η_(inf) for Blends of Hydrocarbon Fractions

Two additional vacuum resid fractions (VR4 and VR5) where characterized to determine T₀ and η_(inf) values by fitting viscosity versus temperature data using Equation (2). Blends of VR4 and VR5 at various weight ratios were then formed. The blends were also characterized (measurement of viscosity versus temperature) to allow for calculation of T₀ and η_(inf) values for each blend. Additionally, the T₀ and η_(inf) values for VR4 and VR5 were used in conjunction with Equation (3) and Equation (4) to calculate curves corresponding to T₀ and η_(inf) values for blends of VR4 and VR5.

FIG. 3 shows a comparison of a) the T₀ and η_(inf) values determined for each blend of VR4 and VR5 based on measured viscosity values at various temperatures, and b) the T₀ and η_(inf) curves predicted by using the T₀ and η_(inf) values for VR4 and VR5 in Equation (3) and Equation (4). As shown in FIG. 3, within reasonable experimental error, the predicted T₀ and η_(inf) from Equation (3) and Equation (4) are in good agreement with the measured values for each blend of VR4 and VR5. This demonstrates the ability to predict the T₀ and η_(inf) values of a modified mesophase pitch fraction if the T₀ and η_(inf) values are known for the individual components used to form the modified mesophase pitch fraction.

Example 3—Blending of Extract and Raffinate to Form Modified Mesophase Pitch

The following example is a prophetic example. In this example it is assumed that the initial feed has T₀=300 K and η_(inf)=8×10⁻⁵ Pa*s. The initial feed is then separated into two streams with different T₀ and η_(inf). Stream 1 has T₀=230 K and η_(inf)=8×10⁻⁵ Pa*s and stream 2 having T₀=315 K and η_(inf)=8×10⁻⁵ Pa*s. Stream 1 corresponds to 18 vol % of the initial feed, while Stream 2 corresponds to 82 vol % of the initial feed. The streams were combined in the volumetric fractions ϕ₁=0.30 and ϕ₂=0.70, producing a modified product with T₀=289.5 K and η_(inf)=8×10⁻⁵ Pa*s.

FIG. 4 show a comparison of the temperature dependence of the viscosity for the initial feed and the modified product. In FIG. 4, the viscosity is plotted on a log scale. As shown in FIG. 4, the modified product has a reduced temperature dependence for all temperatures, including those temperature near 550 K to 595 K that correspond to a desirable temperature range for production of various widths of carbon fiber.

To further illustrate the reduced temperature dependence, a derivative with respect to temperature was taken for the plot shown in FIG. 4. The resulting derivative corresponds to Equation (5).

d log(η)/dT=−0.434*D*T ₀/(T−T ₀)²)  (5)

The T₀ and η_(inf) values for the initial feed and the modified product were used in Equation 5 to show the rate of change of viscosity as a function of temperature. This plot is shown in FIG. 5. As shown in FIG. 5, at sufficiently high temperatures, both the initial feed and the modified product have relatively small temperature dependence for the viscosity, but the modified pitch fraction has a consistently lower temperature dependence than the feed. However, in the temperature region of interest for formation of at least some types of carbon fiber, the initial feed has a substantially greater temperature dependence for the viscosity.

Additional Embodiments Embodiment 1

A method for forming a modified pitch, comprising: performing solvent extraction on a pitch comprising a T₀ value to form an extract comprising a first fraction of the pitch and a raffinate comprising a second fraction of the pitch; and blending at least a portion of the first fraction with at least a portion of the second fraction to form a modified pitch having a modified T₀ value less than the T₀ value of the pitch.

Embodiment 2

The method of Embodiment 1, wherein the first fraction and the second fraction comprise a first volume ratio, and wherein a volume ratio comprising the at least a portion of the first fraction and the at least a portion of the second fraction is greater than the first volume ratio.

Embodiment 3

The method of any of the above embodiments, wherein the at least a portion of the first fraction is formed from the extract without performing an additional solvent extraction step; or wherein the at least a portion of the second fraction is formed from the raffinate without performing an additional solvent extraction step; or a combination thereof.

Embodiment 4

The method of Embodiment 1 or 2, wherein the at least a portion of the second fraction is formed by a method comprising: performing a second solvent extraction on at least a portion of the raffinate to form a second extract comprising a third fraction of the pitch and a second raffinate comprising a fourth fraction of the pitch, the fourth fraction comprising the at least a portion of the second fraction.

Embodiment 5

The method of Embodiment 4, wherein the modified pitch further comprises at least a portion of the third fraction, a volume ratio of the at least a portion of the first fraction, the at least a portion of the third fraction, and the at least a portion of the fourth fraction being different from a volume ratio of the first fraction, the third fraction and the fourth fraction in the pitch; or wherein the modified pitch does not include a portion of the third fraction.

Embodiment 6

The method of Embodiment 1, 2, 4, or 5, wherein the at least a portion of the first fraction is formed by a method comprising: performing a third solvent extraction on at least a portion of the first fraction to form a second extract comprising a fifth fraction of the pitch and a second raffinate comprising a sixth fraction of the pitch, the fifth fraction comprising the at least a portion of the first fraction.

Embodiment 7

The method of Embodiment 6, wherein the modified pitch further comprises at least a portion of the fourth fraction, a volume ratio of the at least a portion of the second fraction, the at least a portion of the fourth fraction, and the at least a portion of the third fraction being different from a volume ratio of the second fraction, the fourth fraction and the third fraction in the pitch; or wherein the modified pitch does not include a portion of the fourth fraction.

Embodiment 8

A method for forming a modified pitch, comprising: performing solvent extraction on a pitch comprising a T₀ value to form an extract comprising a first fraction of the pitch and a raffinate comprising a second fraction of the pitch; performing a second solvent extraction on a) at least a portion of the first fraction orb) at least a portion of the second fraction, to form a second extract comprising a third fraction of the pitch and a second raffinate comprising a fourth fraction of the pitch, wherein the second solvent extraction is performed on the at least a portion of the first fraction, a T₀ of the fourth fraction being less than the T₀ value of the pitch, the modified pitch corresponding to at least a portion of the fourth fraction, or wherein the second solvent extraction is performed on the at least a portion of the second fraction, a T₀ value of the third fraction being less than the T₀ value of the pitch, the modified pitch corresponding to at least a portion of the third fraction.

Embodiment 9

The method of any of the above embodiments, wherein the pitch comprises a mesophase pitch and wherein the modified pitch comprises a modified mesophase pitch; or wherein the pitch comprises an isotropic pitch and wherein the modified pitch comprises a modified isotropic pitch, the mesophase pitch optionally comprising 10 vol % or more of an optically active fraction.

Embodiment 10

The method of any of the above embodiments, wherein the pitch comprises a glass transition having a width of 40° C. or less; or wherein the pitch comprises a softening point of 100° C. to 300° C.; or wherein the pitch comprises a coke value of at least 50 wt. % or more; or wherein the pitch comprises a viscosity at 280° C. of 20 Pa-sec or less; or wherein the pitch comprises a T5 distillation point of 350° C. or more; or wherein the pitch comprises a T50 distillation point of 500° C. or more; or a combination of any two or more thereof; or a combination of any three or more thereof.

Embodiment 11

The method of any of the above embodiments, further comprising separating an extract product fraction and a solvent fraction from the extract, the extract product fraction comprising the at least a portion of the first fraction.

Embodiment 12

The method of any of Embodiments 4-11, further comprising separating a solvent recovery fraction from the raffinate; or further comprising separating a second solvent recovery fraction from the second raffinate; or a combination thereof.

Embodiment 13

The method of any of Embodiments 4-12, wherein performing a solvent extraction comprises performing an extraction with a first solvent, wherein performing a second solvent extraction comprises performing an extraction with a second solvent, and wherein the first solvent has a solubility parameter that is different from a solubility parameter of the second solvent by 1.0 or more, or 2.0 or more, or 4.0 or more.

Embodiment 14

The method of any of the above embodiments, wherein the modified pitch comprises a modified T₀ value that is at least 5.0° C. less than the T₀ value of the pitch, or at least 10° C. less; or wherein the modified pitch comprises a modified η_(inf) value that is greater than a η_(inf) value of the pitch; or a combination thereof.

Embodiment 15

A modified pitch made according to the method of any of Embodiments 1-14.

When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof by those skilled in the art to which the invention pertains.

The present invention has been described above with reference to numerous embodiments and specific examples. Many variations will suggest themselves to those skilled in this art in light of the above detailed description. All such obvious variations are within the full intended scope of the appended claims. 

1. A method for forming a modified pitch, comprising: performing solvent extraction on a pitch comprising a T₀ value to form an extract comprising a first fraction of the pitch and a raffinate comprising a second fraction of the pitch; and blending at least a portion of the first fraction with at least a portion of the second fraction to form a modified pitch having a modified T₀ value less than the T₀ value of the pitch.
 2. The method of claim 1, wherein the first fraction and the second fraction comprise a first volume ratio, and wherein a volume ratio comprising the at least a portion of the first fraction and the at least a portion of the second fraction is greater than the first volume ratio.
 3. The method of claim 1, wherein the at least a portion of the first fraction is formed from the extract without performing an additional solvent extraction step; or wherein the at least a portion of the second fraction is formed from the raffinate without performing an additional solvent extraction step; or a combination thereof.
 4. The method of claim 1, wherein the at least a portion of the second fraction is formed by a method comprising: performing a second solvent extraction on at least a portion of the raffinate to form a second extract comprising a third fraction of the pitch and a second raffinate comprising a fourth fraction of the pitch, the fourth fraction comprising the at least a portion of the second fraction.
 5. The method of claim 4, wherein the modified pitch further comprises at least a portion of the third fraction, a volume ratio of the at least a portion of the first fraction, the at least a portion of the third fraction, and the at least a portion of the fourth fraction being different from a volume ratio of the first fraction, the third fraction and the fourth fraction in the pitch.
 6. The method of claim 4, wherein the modified pitch does not include a portion of the third fraction.
 7. The method of claim 4, wherein performing a solvent extraction comprises performing an extraction with a first solvent, wherein performing a second solvent extraction comprises performing an extraction with a second solvent, and wherein the first solvent has a solubility parameter that is different from a solubility parameter of the second solvent by 1.0 or more.
 8. The method of claim 1, wherein the at least a portion of the first fraction is formed by a method comprising: performing a third solvent extraction on at least a portion of the first fraction to form a second extract comprising a fifth fraction of the pitch and a second raffinate comprising a sixth fraction of the pitch, the fifth fraction comprising the at least a portion of the first fraction.
 9. The method of claim 8, wherein the modified pitch further comprises at least a portion of the fourth fraction, a volume ratio of the at least a portion of the second fraction, the at least a portion of the fourth fraction, and the at least a portion of the third fraction being different from a volume ratio of the second fraction, the fourth fraction and the third fraction in the pitch; or wherein the modified pitch does not include a portion of the fourth fraction.
 10. The method of claim 8, wherein performing a solvent extraction comprises performing an extraction with a first solvent, wherein performing a third solvent extraction comprises performing an extraction with a second solvent, and wherein the first solvent has a solubility parameter that is different from a solubility parameter of the second solvent by 1.0 or more.
 11. The method of claim 1, wherein the pitch comprises a mesophase pitch and wherein the modified pitch comprises a modified mesophase pitch; or wherein the pitch comprises an isotropic pitch and wherein the modified pitch comprises a modified isotropic pitch.
 12. The method of claim 11, wherein the mesophase pitch comprises 10 vol % or more of an optically active fraction.
 13. The method of claim 1, wherein the pitch comprises a glass transition having a width of 40° C. or less; or wherein the pitch comprises a softening point of 100° C. to 300° C.; or wherein the pitch comprises a coke value of at least 50 wt. % or more; or a combination thereof.
 14. The method of claim 1, wherein the pitch comprises a viscosity at 280° C. of 20 Pa-sec or less; or wherein the pitch comprises a T5 distillation point of 350° C. or more; or wherein the pitch comprises a T50 distillation point of 500° C. or more; or a combination thereof.
 15. The method of claim 1, wherein the modified pitch comprises a modified T₀ value that is at least 5.0° C. less than the T₀ value of the pitch, or wherein the modified pitch comprises a modified η_(inf) value that is greater than a η_(inf) value of the pitch, or a combination thereof.
 16. A method for forming a modified pitch, comprising: performing solvent extraction on a pitch comprising a T₀ value to form an extract comprising a first fraction of the pitch and a raffinate comprising a second fraction of the pitch; performing a second solvent extraction on a) at least a portion of the first fraction or b) at least a portion of the second fraction, to form a second extract comprising a third fraction of the pitch and a second raffinate comprising a fourth fraction of the pitch, wherein the second solvent extraction is performed on the at least a portion of the first fraction, a T₀ of the fourth fraction being less than the T₀ value of the pitch, the modified pitch corresponding to at least a portion of the fourth fraction, or wherein the second solvent extraction is performed on the at least a portion of the second fraction, a T₀ value of the third fraction being less than the T₀ value of the pitch, the modified pitch corresponding to at least a portion of the third fraction.
 17. The method of claim 16, further comprising separating an extract product fraction and a solvent fraction from the extract, the extract product fraction comprising the at least a portion of the first fraction.
 18. The method of claim 16, further comprising separating a solvent recovery fraction from the raffinate; or further comprising separating a second solvent recovery fraction from the second raffinate; or a combination thereof.
 19. The method of claim 16, wherein performing a solvent extraction comprises performing an extraction with a first solvent, wherein performing a second solvent extraction comprises performing an extraction with a second solvent, and wherein the first solvent has a solubility parameter that is different from a solubility parameter of the second solvent by 1.0 or more.
 20. The method of claim 16, wherein the pitch comprises a mesophase pitch and wherein the modified pitch comprises a modified mesophase pitch. 